In general, transmission impairments in lightwave systems mainly results from linear and non-linear signal dispersion, noise and intermodulation in case of multiple signals. Dispersion broadens the width of pulses, resulting in inter-symbol-interference (ISI) which limits the maximum bit rate of the system. In particular the mono-mode fiber characteristics of chromatic dispersion caused by frequency dependent delay and attenuation and of polarization mode dispersion (PMD) denoting the propagation of two polarization modes with different frequency dependent signal delays superimposing at the photodiode result in linear and non-linear ISI.
Furthermore, limited laser and receiver bandwidth and an enlarged spectral shape of the transmitted pulses due to a laser phase noise increase ISI.
In the past decades, progress in fiber and laser technologies has reduced many of these impairments considerably, so that in particular for bit rates below 2.5 Gbit/s a threshold detection without equalization may be used in fiber-optic-digital communications since the amount of ISI is small enough to guarantee a high signal-to-noise ratio (SNR) and therefore a bit error ratio (BER) well below 10−12.
However, for bit rates of at least 10 Gbit/s, such as used for example within optical long distance traffic communication systems, the amount of ISI is significantly enlarged. Such communication systems usually using standard-mono-mode fibers, erbium-doped fiber amplifiers and non-coherent optical receivers are still disturbed by non-linear dispersion and signal dependent non-Gaussian noise resulting in a decreased SNR which is additionally decreased by reduced signal power due to wavelength multiplexing or longer coverage distances.
To improve the BER for a given SNR, forward error correcting (FEC) channel coding may be applied. Different FEC coding schemes like BCH-codes (Bose-Chaudhury-Hoequengheen-codes) and RS-codes (Reed-Solomon-codes) are used fitting into the Sonet/SDH digital wrapper format. Based on the redundancy included into the data stream by the encoder at the transmitter-side the decoder at the receiver-side is enabled to correct up a certain amount of transmissions errors. If the number of channel bit errors is below the error correction capability of the FEC code, all bit errors can be corrected, and in addition erroneous channel bits can be marked.
However for ISI, FEC is less suitable. In this regard ISI may be mitigated by an adaptive equalizer at the receiver-side. Well known types are feed forward equalizers (FFE) and decision feedback equalizers (DFE). Particularly, decision-feedback equalization is a widely-used technique for removing inter-symbol-interference where noise enhancement caused by a linear FFE may introduce performance problems. In any case however, for equalizer adjustment, usually an appropriate channel model has to be extracted out of the received analogue signal resulting in an additional expensive circuitry.
In particular in digital transmission systems comprising a receiver including a FFE and a subsequent decision feedback equalizer (DFE) the respective coefficients of the FFE and the respective coefficients of the DFE have to be adjusted separately. This is achieved by extracting an appropriate channel model out of the received analogue signal, thereby needing at least the sign information of the analog signal present at the corresponding tap for each of the adjustment, respectively. Hence, based on the access to the received signal, which has to be analogue-to-digital converted an additional expensive circuitry is required. Additionally, due to slowly time varying distortions, the respective coefficients have to be adapted to the channel permanently.
Moreover, for high rate data transmission of at least 10 Gbit/s a further main problem is to implement an additional sampling device and an analogue-to-digital converter for delivering necessary information for adaptation. The additional signal path however, also results in a loss of signal energy which impairs the main data detection process.